RESEARCH ARTICLE
www.acsami.org
Graphene Polypyrrole Nanocomposite as a Highly Efficient and
Low Cost Electrically Switched Ion Exchanger for Removing ClO4
from Wastewater
Sheng Zhang,†,‡ Yuyan Shao,† Jun Liu,† Ilhan A. Aksay,§ and Yuehe Lin*,†
†
Pacific Northwest National Laboratory, Richland, Washington 99352, United States
School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, China
§
Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey 08544, United States
‡
ABSTRACT: Perchlorate (ClO4 ) contamination is a widespread concern affecting water utilities. In the present study,
functionalized graphene sheets were employed as the scaffold to
synthesize a novel graphene polypyrrole (Ppy) nanocomposite, which served as an excellent electrically switched ion
exchanger for perchlorate removal. Scanning electron microscopy and electrochemical measurements showed that the 3D
nanostructured graphene Ppy nanocomposite exhibited a significantly improved uptake capacity for ClO4 compared with Ppy
film alone. X-ray photoelectron spectroscopy confirmed the uptake and release process of ClO4 in graphene Ppy nanocomposite.
In addition, the presence of graphene substrate resulted in high stability of graphene Ppy nanocomposite during potential cycling.
The present work provides a promising method for large scale water treatment.
KEYWORDS: graphene, polypyrrole, perchlorate, uptake capacity, ion exchange, water treatment
1. INTRODUCTION
Perchlorate (ClO4 ) is a critical component in combat munitions, rocket fuels, and fireworks.1 There is a significant health
concern since ClO4 can block the uptake of iodine in the thyroid
gland and affect the production of the thyroid hormones.2,3
However, due to its high solubility and nonreactivity, ClO4 can
exist in water for many decades. Therefore, ClO4 contamination
is now recognized as a widespread concern affecting water utilities.
It is difficult to remove ClO4 using the conventional separation
technology.4,5 Recently, an electrically switched ion exchange
(ESIX) system has been developed as a green separation technology for removing ClO4 and Cs+ ions from wastewater,2,6 9
where the uptake and release of ions is controlled directly by the
applied potential.
Conducting polymers have attracted considerable attention
due to the promising applications in energy storage/conversion,
optoelectronics, chemical sensors, and separations.9 11 Among
the conducting polymers, polypyrrole (Ppy) and its derivatives
have attracted considerable attention because of their stability
and potential use as an excellent ion exchange material to
selectively remove the ClO4 ions in water.2,9 The ClO4 ion
uptake occurs during electrochemical oxidation of Ppy by
applying an anodic potential which forces ClO4 from the waste
solution into the film. The ClO4 release occurs when the
potential is switched to cathode, forcing ClO4 out of the film
into the elution solution. The Ppy-based ESIX system can be
repeatedly used to significantly reduce the secondary waste.
However, its disadvantage is the limited capacity and the poor
stability for the removal of ClO4 ions. One possible approach is
to employ a substrate2 which can provide many nucleation sites
r XXXX American Chemical Society
for the deposition of Ppy and also act as a substrate to prevent
Ppy film from structural destruction.
Graphene, a single-atom-thick carbon material with large surface
area, superior chemical stability, and low production cost,12 14
could act as an ideal substrate for depositing functional materials for
high-performance electrocatalytic or electrochemical devices.15 19
The graphene Ppy nanocomposites have been synthesized and
used as electrochemical sensors and supercapacitors.20 22 In this
report, to the best of our knowledge, we demonstrate the first use of
graphene Ppy nanocomposite in water purification. The graphene Ppy nanocomposite was prepared by an electrochemical
method using functionalized graphene sheets (FGSs) that can be
produced in bulk quantitites,23,24 which was characterized and
evaluated as a superior electrically switched ion exchanger for
removing ClO4 ions. This makes it more practical to use a low
cost graphene-based composite for large scale water treatment.
2. EXPERIMENTAL SECTION
Functionalized graphene sheet used in this study was prepared
through a thermal expansion process which yields a functionalized
(with epoxy and hydroxyl groups) and a defective form of graphene.23
This functionalized graphene (referred to as graphene in the rest of the
paper) was dispersed in ethanol with the aid of ultrasonication. The
graphene suspension (10 μL, 2 mg mL 1) was applied onto prepolished
glassy carbon (GC) working electrode (5 mm in diameter) and dried at
room temperature. The graphene Ppy nanocomposite was prepared by
Received:
Accepted:
A
June 28, 2011
August 4, 2011
dx.doi.org/10.1021/am200839m | ACS Appl. Mater. Interfaces XXXX, XXX, 000–000
ACS Applied Materials & Interfaces
RESEARCH ARTICLE
Figure 1. Amperometric i t curves for electrodeposit of Ppy films on
the GC electrode and graphene modified GC electrode.
electrochemical polymerization of polypyrrole on the graphene sheets,
which was carried out in a standard three-electrode system controlled
CHI660C station with Pt wire and Ag/AgCl as the counter and
reference electrodes, respectively. The Ppy film was deposited on
graphene modified GC electrode by a constant potential of 0.7 V vs.
Ag/AgCl for a period of 6 min in an N2-purged aqueous solution
containing 0.125 M pyrrole monomer and 0.2 M NaCl. For comparison,
the Ppy film on the bare GC electrode was synthesized via the same
method. After electropolymerization, the modified working electrode
was completely rinsed with deionized water.
Scanning electron microscopy (SEM) images were obtained with an
FEI Helios Nanolab dual-beam focused ion beam/scanning electron
microscope (FIB/SEM) operated at 2 kV. X-ray photoelectron spectroscopy (XPS) measurements were made using a Physical Electronics
Quantum-5600 Scanning ESCA Microprobe. Al X-ray source was operated at 250 W. The sample to analyzer takeoff angle was 45°. Survey
spectra were collected at a pass energy (PE) of 187.85 eV over the
binding energy range of 0 1300 eV. High binding energy resolution
Multiplex data for the individual elements were collected at a PE of
29.55 eV.
3. RESULTS AND DISCUSSION
The amperometric i t curves during the electrochemical
deposition of polypyrrole at the bare GC electrode and the
graphene modified GC electrode are shown in Figure 1. In
contrast to the case of GC electrode, a distinct current peak
within the first seconds is observed for the graphene modified
GC electrode. This suggests different nucleation processes in the
initial stages of Ppy electropolymerization:25 most likely, a higher
number of nucleation sites is generated on graphene modified
GC electrode compared to the bare GC electrode which can be
attributed to the high surface area and electronic conductivity of
graphene nanosheets.26 The SEM image in Figure 2A shows the
graphene sheets of a few micrometers in lateral size with the
characteristic lamellar structure of graphene.27 Figure 2B,C
shows typical Ppy nanostructures containing spherical particles
with diameters ranging from 200 to 800 nm for both Ppy and
graphene Ppy. However, Ppy film on GC electrode (Figure 2B)
is very compact and lacks nanoporosity, while the Ppy film in
graphene Ppy nanocomposite (Figure 2C) is loose and contains many spherical particles creating a 3D porous nanostructure
which can be highly beneficial for ion diffusion.28 Commensurate
with these characteristics, below, we demonstrate that the
graphene Ppy nanocomposites indeed exhibit a high performance for the removal of ClO4 ions.
The electroactivity and the reversible redox properties of Ppy
have attracted great interest in the development of electrochemically
Figure 2. SEM images of graphene (A), Ppy (B), and graphene Ppy (C).
controlled delivery devices and separation systems for charged
species.29,30 When Ppy films are cycled in appropriate electrolytes, the reversible redox switching of the film is accompanied by
the movement of counterions in and out of the polymer film for
charge balance. Figure 3 shows the schematic illustration of the
electrically controlled anion exchange for the separation of the
perchlorate ion from wastewaters using Ppy film as the electroactive ion exchanger. The ClO4 ion uptake occurs when the
electrochemical oxidation of the electroactive species is performed by applying an anodic potential on the film in the solution
containing ClO4 which forces the ClO4 from the waste
solution into the film. Correspondingly, the ClO4 ion release
occurs when the potential is switched to cathode which forces the
ClO4 out of the film and into the elution solution. To maintain
the charge balance, the charges during the oxidation and reduction of Ppy films are compensated with the same amount of
ClO4 into and out of Ppy films.
Cyclic voltammetry is a useful tool to evaluate the capacity of
electroactive materials. Figure 4 shows the cyclic voltammograms
(CVs) of graphene, Ppy film, and graphene Ppy films in 0.2 M
NaClO4 solution. The CV on graphene shows the rectangular
and symmetric current potential characteristics, and there are
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dx.doi.org/10.1021/am200839m |ACS Appl. Mater. Interfaces XXXX, XXX, 000–000
ACS Applied Materials & Interfaces
RESEARCH ARTICLE
Figure 3. Schematic illustration for the uptake and release process of ClO4 with the oxidation and reduction of Ppy films.
Figure 4. Cyclic voltammograms (2 mV s 1) of Ppy and graphene
Ppy films in 0.2 M NaClO4 solution.
Figure 6. XPS survey (A) and high resolution Cl spectrum (B) of asprepared graphene Ppy nanocomposite (a) and after controlling the
electrode at 0.4 V for 600 s (b) and at 0.8 V for 600 s (c) in a solution
containing 0.2 M NaClO4.
broad reduction peak are observed on both Ppy and graphene
Ppy films which are characteristic of Ppy films.31,32 The ion
uptake and release can be controlled by modulating the electrode
potential: during the oxidation, cationic sites such as polarons
(Ppy+) are formed on the Ppy film, and at the same time, the
anion species such as ClO4 in the solution are forced into the
Ppy film to balance the positive charges; correspondingly,
the ClO4 ions are released from Ppy film during the reduction process. A much higher oxidation peak current can be
observed on the graphene Ppy film which is 2.5 times higher
than that of the Ppy film. It is obvious that the charges during
the oxidation of graphene Ppy film are much higher than
that of the Ppy film. These charges during the Ppy oxidation
are compensated with the same amount of ClO4 ions into
the Ppy film or graphene Ppy film. We, therefore, conclude
that the graphene Ppy film has a much higher uptake capacity
for ClO4 than the Ppy film. The possible reason for this
Figure 5. i t curve during the anodic (0.4 V) and cathodic ( 0.8 V)
treatment in 0.2 M NaClO4 solution.
no clear redox peaks observed in the potential range between
0.8 and +0.4 V. However, a relatively sharp oxidation peak and a
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dx.doi.org/10.1021/am200839m |ACS Appl. Mater. Interfaces XXXX, XXX, 000–000
ACS Applied Materials & Interfaces
RESEARCH ARTICLE
The stability of Ppy film is one of the key factors for its
application as an ion exchanger. The consecutive CVs shown in
Figure 7 indicate that the loss of capacity for ClO4 exchange
occurs quickly on repeated cycling for the Ppy film on the bare
GC electrode. In contrast, the oxidation peak current on
graphene Ppy film keeps constant during the consecutive
cycling. In addition, the oxidation peaks gradually shift negatively
for both Ppy and graphene Ppy films which is due to the
structural relaxation of Ppy film with potential cycling.2 The
negative shift of the peak potential on the graphene Ppy film is
much smaller than that on the Ppy film, indicating less structural
change on the graphene Ppy flim. These results indicate that
the stability of the Ppy film is significantly improved when it is
deposited on the surface of graphene which can be attributed to
the possibility that the large particle-sized graphene substrate
prevents the structural change of Ppy film.
4. CONCLUSIONS
Functionalized graphene polypyrrole nanocomposites were
processed using an electrodeposition method. The nanocomposite improved the capacity for ClO4 exchange which we
attribute to its 3D porous nanostructure facilitating an easy
ClO4 diffusion into or out of the Ppy film. Further, the presence
of graphene substrate resulted in an improved stability of Ppy
film during potential cycling. Therefore, our present study
provides a green process for removing ClO4 from wastewater
through an electrically switched ion exchange. Moreover, this
novel graphene Ppy film may find applications in other fields,
such as supercapacitors and chemical sensors.
Figure 7. Consecutive cyclic voltammograms for Ppy and graphene
Ppy films in 0.2 M NaClO4 solution. The scan rate: 5 mV s 1.
higher uptake is that the 3D porous nanostructure of the
graphene Ppy film facilitates ClO4 to easily diffuse into or out
of the film.
The process of electrically switched anion exchange for ClO4
ions may proceed at a higher rate. The i t curves in Figure 5,
recorded during the anodic and cathodic treatment for graphene
Ppy film, exhibited that the process of electrically switched anion
exchange almost finished within 20 s. XPS was employed to
provide further evidence for the uptake and the release process of
ClO4 in graphene Ppy nanocomposite. Figure 6 shows the
XPS spectrum of the as-prepared graphene Ppy nanocomposite
(line a) and after different electrochemical treatments (lines b
and c). As shown in Figure 6A(a),B(a), the appearance of peaks
around 200 eV indicates that Cl was doped into Ppy film in
graphene Ppy nanocomposite during the polymerization.
Figure 6A(b),B(b) shows the XPS data of graphene Ppy
nanocomposite after an applied anodic potential of 0.4 V for
600 s in a solution containing 0.2 M NaClO4. The peaks around
210 eV can be distinguished which means that the ClO4 has
been taken into the Ppy film in the graphene Ppy nanocomposite under an anodic potential. Then, the graphene Ppy nanocomposite was cathodically polarized at 0.8 V for 600 s in a
solution of 0.2 M NaClO4. As shown in Figure 6A(c),B(c), the
peaks of Cl 2p disappeared, indicating that almost all the ClO4
anions were ejected out of the film which reflects the feasibility of
electrically controlled anion exchange. The Cl ion is spherical
with a radius of 0.181 nm, while the shape of ClO4 is tetrahedral
with a size of 0.240 nm. These differences in shape and size will
exert an influence on their strength of interaction with Ppy chains
and also on their mobility in and out of the Ppy films. Studies
confirmed that the mobility of ClO4 was smaller than that
of Cl , which results in the preferential ClO4 selectivity over
Cl for Ppy film.2
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
’ ACKNOWLEDGMENT
The work was done at Pacific Northwest National Laboratory
(PNNL) and was supported by the U.S. Department of
Defense’s SERDP environmental research program (Project ER1433). The characterization was performed using Environmental
Molecular Sciences Laboratory, a national scientific-user facility
sponsored by the DOE’s Office of Biological and Environmental
Research and located at PNNL. PNNL is operated for DOE by
Battelle under Contract DE-AC05-76RL01830. The authors
would like to acknowledge Dr. Laxmikant Saraf for SEM
measurement. S.Z. acknowledges a fellowship from the China
Scholarship Council and PNNL to perform this work at PNNL.
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